1. Field of the Invention
This invention relates to a method for processing a color image and an apparatus therefor, particularly to a method of color image processing for converting color original image data into desired color copy image data and an apparatus therefor.
More particularly, this invention relates to a color image processing method for faithfully reproducing the color image of an input color original image in the form of a color copy image and to a color image processing apparatus for executing the color image processing method by the use of a small memory region.
2. Description of the Prior Art
Recently, color image processing apparatuses for performing the operation of producing color copy images on recording media based on input color original image data have begun to be disseminate in various fields. In the field in which the color image processing apparatuses are utilized as ink jet printers, for example, because of the high excellence in color reproducibility, they find utility mainly in applications specializing in designs of apparel and room interiors, namely the applications which demand faithful color reproduction of original images.
When a color, image given as an original image on a color CRT display and a color image printed out as a copy image by an ink jet printer are compared, they are generally found to lack conformity between the range of reproduction as to color saturation and luminosity of the original image, namely the range of color reproduction, and the range of reproduction of the copy image. For faithful reproduction of the color image of the original image on the reproduced image, therefore, the color image signal being fed into the ink jet printer during the course of the reproduction should be corrected. The technique disclosed in U.S. Pat. No. 4,758,885, for example, effects faithful reproduction of the color image of an original image on a copy image where the range of color reproduction of the color image signal in the input system is wider than that of color reproduction in the output system by causing the input color image to be copied with the range of color reproduction in the input system compressed to that in the output system. This technique, compared with the conventional technique which gives no correction of any sort to the input color image signal, permits reproduction of a copy image rich in gradation.
The image processing apparatus which executes the conventional method of color image signal processing mentioned above is required to carry out the processing of high intricacy at a high speed. As means of expediting this processing, Japanese Patent Application Disclosure SHO 59(1984)-123,390, for example, teaches a technique which comprises preparing a lookup table storing data necessary for the processing, retrieving pertinent data from the lookup table, and enabling the processing to be speedily carried out based on the output data. When this technique relies solely on the provision of this lookup table, the lookup table is inevitably required to possess a very large capacity to a degree to jeopardize the economy of the technique itself. To reduce the volume of data stored in one lookup table, therefore, the technique is compelled to make use of a plurality of such lookup tables. In the performance of hard copying based on an input color image signal, a particular lookup table suitable for the quality of the input color image signal is automatically selected from the plurality of lookup tables and an image of ample legibility is formed based on the contents of the selected lookup table. By this technique, the expediting of the processing and the repression of the rise of cost are both satisfied.
While the conventional image processing apparatuses described above are capable of producing copy images rich in gradation, the copy images they produce have the possibility of portraying a deficiency in naturalism.
This disadvantage is ascribed to the fact that the conventional method of color image signal processing takes due account exclusively of the conformation of the range of color reproduction between the original image and the copy image and pays no consideration to the white point or the achromatic axis.
In accordance with the conventional method of color image signal processing, since it does not take into consideration the conformation of the white point or the achromatic axis, there are times when the gradation is represented while entailing deviation of white level or color hue. When a white color is to be represented, therefore, the white color produced appears with a grayish tint. When a gray color is to be represented, the produced white color appears with a reddish tint or a bluish tint. As a result, the copy image entails the phenomenon of showing a fringe pattern thereon and is possibly deprived conspicuously of naturalism.
An effort to represent an original image in multi-gradation results in a colossal addition to the volume of data to be stored in the lookup table for the correction of multi-gradation images and calls for provision of a large memory region. An effort to store in a memory a multiplicity of lookup tables fit for the status of the input system, for example, necessitates provision of a large memory region.